Applying balanced line technology to the F1-600MR.

Updated: 29 April 2001
To achieve the best possible performance in an audio system, it is necessary to ensure that ground loops do not degrade the noise floor of the system. Such loops normally introduce a hum or buzz; it is our position that no such situation is ever acceptable, and we have therefore gone to great lengths to minimise the possibility that any piece of TAG equipment should either create a ground loop or be susceptible to the effects of one. The problem is not particularily simple; an audio system may consist of many units connected together, only one of which the designer has any influence over. Nonetheless there are certain techniques that can improve ground-loop immunity in any situation, and we have made full use of them in the design of the F1-600M power amplifier.

The F1-600M amplifier is a Class-I piece of equipment; in other words the metal chassis is connected to mains ground. In this respect it differs from most TMA products, which are Class-II or "double-insulated", that is, with no connection to mains earth. There are very good technical and sonic reasons for adopting the Class-I approach in a high-power amplifier, and special care has therefore been taken that the F1 will be specially resistant to non-ideal system conditions, and will certainly not introduce any problems itself.

The first section of this article described the three main ways in which potentially problematic ground currents could be generated in an audio system, and the two ways in which these currents could degrade performance. All of these possibilities have been dealt with in the F1-600M design.

1: Mains grounding currents. These cannot be generated by electronic equipment alone.

2: Transformer stray magnetic fields. The F1-600M mains transformer is a toroidal design which minimises external fields. The internal topology and construction of the amplifier prevents what traces of field that do exist from interacting with the ground wiring.

3: Transformer stray capacitance. The mains transformer in a powerful amplifier is inevitably large, with relatively big stray capacitances between primary and secondary. The potential problems this could cause are suppressed by a grounded electrostatic screen that completely separates the primary and secondary windings. This screen also effectively prevents RF noise entering the audio circuitry from the mains supply.

If a ground loop does exist, with enough ground current flowing to cause problems, it may have its effect either outside or inside a unit:

1: Outside effects are caused by ground currents flowing down the shields or cold conductors of an unbalanced audio interconnect, generating voltage drops that add hum to the signal. The voltage-drop down a cable screen or shield can be removed by the use of a balanced input. The F1-600M has such a balanced input, accessed via a professional-quality XLR connector. The balanced input stage itself uses high-precision resistors throughout to ensure high common-mode rejection. (A balanced input works by subtracting the unwanted noise from the signal; common-mode rejection is a measure of how accurately that subtraction is performed, and therefore of how completely the noise is rejected)

2: Inside effects. If ground currents are flowing, it is vital to keep them out of the internal circuitry of the equipment. Internal cabling or PCB tracking is likely to have a higher resistance than than the external interconnects, and the hum problem will be correspondingly worse.
The F1-600M has both a balanced and unbalanced input. If only one is in use then the ground current will flow through the interconnect shield and down the F1-600M mains earth connection. The harm this can do is prevented by means of a sophisticated grounding topology that captures the unwanted current the moment it enters the F1-600M enclosure, and routes it directly back out of the unit, so ground currents and wanted signals never encounter each other.
It may be that both inputs are permanently connected to separate source equipment; the rejection of the unselected input is so great that this is entirely practicable even if both sources are active at once. There is thus the possibility that ground current could be flowing from Source A, through the interconnect to the F1-600M, and from there to Source B. Clearly it must flow through the ground pin of the XLR to the ground collar of the phono (RCA) socket. These two points are therefore joined extremely firmly, the topology of the connection ensuring that once more, ground currents and signal never mingle.